1. Technical Field
The present invention relates to switching converters and to specifically a circuit layout of four switches
2. Description of Related Art
The thermal resistance of materials used to package electronic components is of great interest to electronic engineers, because most electrical components generate heat and need to be cooled. Electronic components need to be cooled to avoid premature aging and consequent failure. Also, effective cooling of the electronic component(s) susceptible to generating heat in a circuit allows for a stable, efficient and predictable performance of the circuit. In particular, heat generated from electronic components in power supply/conversion circuits are mostly derived from the main switching devices.
Heat sinks function by efficiently transferring thermal energy or heat from a first object at a relatively high temperature to a second object or the environment at a lower temperature with a much greater heat capacity. This rapid transfer of thermal energy quickly brings the first object into thermal equilibrium with the second object or environment, lowering the temperature of the first object thus fulfilling the role of a heat sink as a cooling device.
FIGS. 1a and 1b show a plan and side view respectively of a circuit board 100 with heat sink 102 according to conventional art. Four switches 104 are shown. Switches 104 are electrically connected to circuit board 100 via legs 106. Plate 104a is used to mechanically attach switch 104 to heat sink 102 using threaded screw 108. An application of a heat sink compound (typically made from zinc oxide in a silicone base) is applied between plate 104a and heat sink 102 prior to fastening with threaded screw 108. The heat sink compound allows for better heat transfer from switch 104 and heat sink 102 to allow for the uneven surfaces of either plate 104a or heat sink 102. Typically switch 104 is a semiconductor switch such as a metal oxide semi-conductor field effect transistor (MOSFET) or insulated gate bipolar transistor (IGBT).
FIG. 2a shows a conventional full bridge converter 20. Full bridge DC to DC converter 20 has four main switches S1, S2, S3 and S4 connected together in a full bridge configuration. Switches S1, S2, S3 and S4 are insulated gate bipolar transistors. The collectors of switch S1 and switch S3 are connected together at node Y1 and the emitters of switch S2 and switch S4 are connected together at node Y2. The emitter of switch S1 is connected to the collector of switch S2 and the emitter of switch S3 is connected to the collector of switch S4. Each of the four main switches (S1, S2, S3 and S4) has respective diode shunts (D1, D2, D3 and D4) connected in parallel thereto. The diode shunts may be inherent parasitic diodes of the IGBTs, or may be discrete components. The diodes placed across switches S1 and S2 are in both the same direction similarly the diodes of switch S3 and switch S4 are both in the same direction. In the case where full bridge converter 50 is operated as a DC-to-DC converter all diodes (D1, D2, D3 and D4) connected across switches S1, S2, S3 and S4 are reverse biased with respect to the input voltage Vin. An input voltage (Vin−) of full bridge converter 20 is connected across the node (Y2) between switches S2 and S4 and an input voltage (Vin+) is connected at the node (Y1) between switches S1 and S3. An output voltage (Vout−) of full bridge converter 20 is connected across the node (X1) between switches S1 and S2 and output voltage Vout+ is connected at the node (X2) between switches S3 and S4. Switching of full bridge converter 20 is typically done in a manner such that while switches S1 and S4 are ON, switches S3 and S2 are OFF and vice versa.
FIG. 2b shows a typical conventional buck-boost DC-to-DC converter circuit 22. The buck circuit of buck-boost DC-to-DC converter 22 has an input voltage Vin with an input capacitor C1 connected in parallel across Vin. Two switches are implemented as field effect transistors (FET) with integral diodes: a high side buck switch Q1 and a low side buck switch Q2 connected in series by connecting the source of Q1 to the drain of Q2. The drain of Q1 and the source of Q2 are connected parallel across an input capacitor C1. A node is formed between switches Q1 and Q2 to which one end of an inductor 206 is connected. The other end of inductor 206 is connected to the boost circuit of buck-boost DC-to-DC converter 22 at a second connecting two switches: a high side boost switch Q4 and a low side boost switch Q3 together in series where the source of Q4 connects to the drain of Q3 to form node B. The drain of Q4 and the source of Q3 connect across an output capacitor C2 to produce the output voltage Vout of buck-boost DC-to-DC converter 22.
At higher switching frequencies of switched inverters/converters, lower values of reactive components can be used in circuit to achieve the required output characteristics of the inverters/converters. However, the increase in frequency can have the undesirable effect of increasing electromagnetic interference (EMI) if good circuit design and good circuit layout practices are not followed. Remembering that currents flowing in a closed path, i.e. a loop (formed by circuit board traces) acts as an efficient radiator of electromagnetic energy, maximum radiation efficiency occurs when the loop dimension is on the order of one-half wavelength. To minimize the radiation efficiency, that is to reduce radiated noise, the loop is made as physically small as possible by being aware of parasitic inductances in the board traces. High-frequency currents follow the path of least impedance (and not the path of least resistance) and a way to reduce the inductive impedance (XL=2πfL) of parasitic inductances (L) is to reduce the frequency (f) or to reduce the size of the loop, since a longer loop gives more parasitic inductance (L). Power loss (P) in the loop is the product of the inductive impedance (XL) squared and the high frequency current in the loop.
Both static and dynamic power losses occur in any switching inverter/converter. Static power losses include I2R (conduction) losses in the wires or PCB traces, as well as in the switches and inductor, as in any electrical circuit. Dynamic power losses occur as a result of switching, such as the charging and discharging of the switch gate, and are proportional to the switching frequency.